Energy absorption materials are widely used to protect people and goods from damaging impacts and forces. In an impact or blast event these materials should reduce the impulsive load to a level below a damage threshold by absorbing a maximum of energy while not transmitting a stress in excess of the damage threshold. Examples from the automotive, sporting and defense sectors include crash absorbers, helmet pads and blast-mitigating foot pads. Depending on the application, different performance characteristics are required of the energy absorbing material. The injury criterion or damage threshold σth determines the maximum allowable stress, σtr, transmitted through the energy absorber, i.e., to avoid damage is it necessary that σtr<σth. For energy absorbers in direct contact with the human body the injury criterion is generally on the order of 1 MPa.
Cellular materials are often used as energy absorption materials because they can absorb energy on compression. Single use energy absorption materials may be metallic and include closed or open cell foams and pre-crushed honeycombs; multi-use materials with reversible energy absorption are typically polymeric and include visco-elastic closed or open cell foams, and thermoplastic polyurethane (TPU) twin hemispheres. Lattice structures can be composed of polymer or metallic materials and may consist of a periodic arrangement of solid or hollow members (struts, trusses).
FIG. 1 is a schematic plot illustrating the ideal behavior of an energy absorption material. At low stress, the strain increases linearly with stress, up to a stress of, e.g., 1 MPa. At this threshold stress, the strain of the material increases rapidly at substantially constant stress, and the material absorbs energy. Finally, when the material reaches a high strain referred to as the densification strain, the stress again increases. The maximum possible volumetric energy absorption for a given material structure is calculated as the product of the peak stress with 100% strain.
Real materials typically deviate from the ideal response and have lower absorption efficiency. FIG. 2 illustrates the typical behavior of a lattice or truss structure with high structural symmetry and internal connectivity. Here, after reaching a peak initial stress (labeled as Max. transmitted stress in FIG. 2), the strain increases at a lower level of stress, resulting in a reduction in energy absorption compared to the ideal case illustrated in FIG. 1. This is believed to be due to the fact that the onset of buckling at a single point in such a structure with high structural symmetry and internal connectivity triggers buckling throughout the structure, which leads to an abrupt loss of load-carrying capability and reduced impact energy absorption efficiency. In such a case, the densification strain is defined as the strain at which the stress-strain curve intersects a horizontal line at the peak initial stress value. The volumetric energy absorbed is calculated as the area under the stress-strain curve between 0% strain and the densification strain. The energy absorption efficiency of such a material is calculated as the ratio of the volumetric energy absorbed to the maximum possible volumetric energy absorption.
FIG. 3 shows a compressive stress-strain response typical of types of aluminum foam used as energy absorption materials. Such materials have a plateau-like stress-strain curve and do not exhibit the non-ideal behavior of an initial stress peak followed by softening. However, such materials have a low densification strain since the stress starts to rise at an increasing rate at approximately 30% strain, which limits the energy absorption efficiency to be about 35%. FIG. 4 shows the compressive stress-strain response of an aluminum honeycomb energy absorption material. In this case, the material exhibits a large initial stress peak followed by softening, which limits its energy absorption efficiency to about 34%. FIG. 5 shows the compressive stress-strain response of a twin hemisphere energy absorption material. The material exhibits softening after an initial stress peak, which limits its energy absorption efficiency to about 47%.
Certain materials with a truss or lattice architecture have constant architectural parameters through the thickness direction, i.e., the energy absorbing direction of the truss or lattice structure. In these materials, the high structural symmetry and lack of disconnected internal members lead to simultaneous buckling and a sharp loss of load transfer capability as shown in FIG. 2. This reduces the energy absorption efficiency of the material as the stress level associated with compaction drops well below the peak value. FIG. 6 displays the compressive stress-strain response of a typical hollow metallic micro-truss structure with no enhancement on the structure. It exhibits a large stress peak followed by softening, and additional stress peaks. The energy absorption efficiency is about 15%-30%.
Therefore, there is a need for micro-truss or lattice architectures with the inherent structural and low mass benefits of such architectures, yet with improved energy absorption response.